1. Field of the Invention
This invention relates to a system and method for eliminating or at least reducing the amount of unwanted electronic mail messages sent to users of e-mail.
2. Description of Related Art
As the use of electronic mail (e-mail) has grown, so too has the amount of unwanted e-mail. This unwanted e-mail, commonly referred to as “junk e-mail” or “spam,” is usually, but not always, some form of solicitation to buy some product or service.
Junk e-mail in the “Inbox” of e-mail programs is analogous to the “junk mail” that fills many mail boxes. The analogy is not perfect, however. Perhaps the greatest difference, besides the obvious difference in media, is that the marginal cost of sending junk e-mail to one more recipient is essentially nil, whereas an address label must be printed and postage must be paid for each additional piece of junk mail sent through normal postal channels. Indeed, the alarming growth of the junk e-mail industry depends on this cost efficiency. Even millions of e-mail addresses can be obtained relatively cheaply, especially since many organizations that have complied such lists for other reasons are often happy to get the “free” money that is to be had simply by selling them. Even absent such straightforward purchase, less scrupulous “junk e-mailers” can still get at addresses by using known computer programs that gain access to and download address lists from servers. Even more disturbingly, junk e-mailers can also launch programs that cause central e-mail servers themselves to propagate the junk e-mail to their own customers.
That junk e-mail is almost universally hated is of little concern to its creators, once again because of the negligible cost: Assume that 9,999 out of 10,000 who receive an e-mail solicitation for a $100 service are irate at having received it at all, but that one actually buys the service. If the junk e-mailer manages to transmit just a million junk e-mails with the solicitation, then he will still have made $10,000, which is typically far more than it would cost to generate the solicitation. Moreover, note that there are even on-line companies that specialize in sending junk e-mail on behalf of others, usually for only a few hundred dollars, thereby reducing the cost of sending mass junk e-mail even further. Even these companies send junk e-mail advertising their service, which constitutes junk e-mail that advertises a service to enable others to send junk e-mail!
It is not just countless recipients of junk e-mail who would like to rid the e-mail system of it: The hundreds of millions of junk e-mail messages sent every day (and perhaps every hour or minute) represent a waste of the bandwidth of the network (usually Internet) and thus a loss to e-mail service providers as well.
There are at present many different systems that attempt to filter out junk e-mail. Some of these systems are incorporated in the user's e-mail program itself, others are installed in the central e-mail server, and still others are separate applications that the user must load into his computer.
Using perhaps the most common system, when a user receives unwanted e-mail, he may direct his e-mail application to put the sender's address in a “blocked” list. Future messages from the same address are then prevented from reaching the user's Inbox, and are instead sometimes routed to a special junk e-mail folder for optional review by the user. The weakness of this filtering method is that junk e-mailers can easily change the address but keep the same domain name. The junk e-mail will then pass this simple filter. Thus, the sender address “aaa@junk.ext” might be blocked, but “aab@junk.ext” will not be.
One improvement over simple address filtering is therefore to filter based on domain names. There are two weaknesses to this approach. First, it is possible to change the “Reply to” or “From” address of a sent e-mail message. Automatically generated, fake domain names can therefore be sent along with junk e-mail to bypass domain name-based filters. Second, junk e-mailers often misappropriate common domain names, especially if the junk e-mail is being sent by a “Trojan Horse” program from within a widely used e-mail server; users may not want to block all e-mail from that domain.
Other known junk e-mail filters examine the routing information that accompanies each e-mail in an attempt to identify the original address from which the e-mail was sent. This will still not work when the e-mail is sent from within a central “trusted” server. Even so, junk e-mailers may also use known techniques to cause their messages to be sent through different proxy servers and thereby to disguise their origin.
The “Active SMTP” technology of Escom Corp. represents another approach, according to which a sending e-mail host is probed while it is connected to the recipient's server. If the sending host is a dial-up risk, or an open relay, or is configured not to accept incoming e-mail (typically indicative of a forged address), then the Active SMTP system distrusts the sending host and rejects its e-mail. This system has at least two specific weaknesses: First, it will not block unwanted e-mail such as blatant commercial advertisements if the sender is using regular e-mail sent from a properly configured mail server. Second, this system works only when the sending server is connected to the recipient server; consequently, such a system is not suitable for installation on users' local computer systems that download e-mail from a remote server.
Still other known junk e-mail filters examine not just addresses and routings, but rather (or in addition) some aspect of the content of the messages. Content is usually found either in the “subject line” of the message, or the main text of the message, or both. It may also be found in the message header. Assume such a filter is installed in a central e-mail server. If a filter program notices that a million pending e-mail messages all have the same subject line, or have message texts with identical wording, then it is almost certain that these messages are junk e-mail and can be blocked and deleted. To defeat such filters, junk e-mail senders typically add random strings of characters in the subject line (usually after many blank spaces or asterisks to make them less obvious to recipients) or at the bottom of the texts. Character-by-character or word-by-word comparison of any two of the junk messages will therefore show them to be different, although the only difference is the meaningless, random character strings included specifically to defeat the filters.
Other “tricks” used by senders of junk e-mail include fake personalization of the messages. Thus, by including all or some part of the recipient's name (extracted from his e-mail address or from a bulk address list) in the subject line, the sender attempts to fool the recipient into believing that the message is legitimate. For example, the text “Bob, here is the deal I told you about!” might be in the subject line of a message sent to Bob@mailfront.com. This ruse alone may defeat some known filters and may induce the unwitting recipient to open the message. This trick of false familiarity is of course also well known from junk mail sent by post. Note that many of the methods used by junk e-mailers resemble those used to transmit computer viruses, which often require that recipients be tricked into opening some executable attachment.
Legislation has done little to combat the plague of junk e-mail. Indeed, there is proposed federal legislation against “unsolicited commercial e-mail,” but it lacks provision for effective enforcement and has loopholes through which junk e-mailers easily slip. Like laws, evasion is often a simple exercise in semantics. For example, many junk e-mail messages include false or at best misleading statements indicating that they are not unsolicited, or not commercial. Some particularly brazen junk e-mail even states explicitly that it complies with the very proposed legislation it violates. All such statements are of course intended to confuse recipients or at least make them believe they have no recourse. Examples of actual statements received by the applicant's attorney include:                “Below is the result of your feedback form.” (No feedback form was ever submitted.);        “Note: this is not a spam e-mail. This e-mail was sent to you because your e-mail was entered in on (sic) one of our websites requesting to be a registered subscriber.” (No such request had ever been submitted.)        “THIS IS NOT AN OFFER OR CONTRACT TO BUY INSURANCE PRODUCTS, but rather a confidential informational inquiry” (attempting to circumvent the prohibition against unsolicited commercial e-mail); and        “This message is being sent in compliance of (sic) the e-mail bill: Section 301.per section, paragraph (a) (2) (c) of S. 1618.”        
Still other junk e-mail filters work by searching the message for certain predetermined, prohibited keywords. Any messages that contain any of the keywords are blocked and are not passed through to the user's e-mail application. These filters thus operate in the same manner as (and often are) known filters designed to eliminate pornography or other objectionable subject matter. The problems of over- and under-inclusiveness of these systems are well documented.
The general weakness of all of the known junk e-mail filters mentioned above stems from the fact that they are passive: They attempt to determine whether a message is junk e-mail based solely on an analysis of information extracted from and relating to the message and its sending server. As such, the senders of junk e-mail, like car thieves working to defeat alarms, can evolve their techniques so as to defeat the latest filters.
What is needed is a mechanism for eliminating or at least greatly reducing the successful transmission of unwanted e-mail while still making it easy and convenient to receive wanted e-mail. In other words, e-mail from unwanted senders should be effectively blocked, with as little interference as possible with e-mail from approved senders. It should also be impossible—or at least exceptionally difficult—for junk e-mailers to bypass the filter. This invention provides such a mechanism.